Cold rolled recovery annealed mild steel and process for manufacture thereof

ABSTRACT

A high strength mild steel alloy is provided. In addition, a process for making the high strength steel alloy is also provided. The process includes providing a mild steel alloy with a chemical composition in weight percent within a range of 0.12-0.25 carbon, 0.30-1.70 manganese, 0.50 max silicon, 0.10 max chromium, 0.01 max niobium, 0.035 max titanium, 0.01 vanadium, 0.10 max molybdenum, 0.10 max nickel, 0.015 max sulfur, 0.025 max phosphorus, 0.012 max nitrogen, 0.003 max boron, and 0.015-0.065 aluminum. Hot rolled steel strip with a thickness of less than 10 millimeters is cold rolled to produce a cold rolled steel sheet that has a thickness that is less than 50% of the hot rolled steel strip thickness which is subsequently recovery annealed to provide sheet material having a yield strength greater than 550 megapascals (MPa) and a percent elongation to failure greater than 3.5%.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. Provisional Application No. 61/712,326 filed on Oct. 11, 2012, the contents of which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

The use of cold rolling to increase strength levels of carbon steel alloys is known. However, cold rolling can also result in unacceptable low ductility levels of the material. As such, recrystallization annealing in a continuous annealing line (CAL) is known to be used to improve the ductility of cold rolled steels. But, recrystallization annealing and the resulting increase in ductility typically results in a significant loss in yield and tensile strength of the material. Therefore, and in order to maintain desired yield and tensile strength levels, prior art steels have used the addition of alloying elements with an associated increase in material cost.

An alternative method to the use of more expensive alloyed steels is to use batch anneal processing in order to perform a recovery anneal on a low alloy mild steel. Such processing can produce strength levels greater than 550 megapascals (MPa) with ductility greater than 3.5% elongation for low alloy carbon steels. However, batch anneal processing takes a relatively long time compared to CAL processing and also results in temperature non-uniformity across the width of a coil being annealed. Therefore, a significant variation in properties from edge to edge of the coil is known to be a problem.

Given the above, a more cost-effective alloy and efficient process to produce a cold rolled carbon steel that has a strength of greater than 550 MPa and a ductility greater than 3.5% elongation would be desirable.

SUMMARY OF THE INVENTION

A high strength steel alloy is provided. In addition, a process for making the high strength steel alloy is also provided. The process includes providing a steel alloy with a chemical composition in weight percent within a range of 0.12-0.25 carbon, 0.30-1.50 manganese, 0.50 maximum (max) silicon, 0.10 max chromium, 0.01 max niobium, 0.01 max titanium, 0.01 vanadium, 0.10 max molybdenum, 0.10 max nickel, 0.015 max sulfur, 0.025 max phosphorus, 0.012 max nitrogen, 0.003 max boron, and 0.015-0.065 aluminum. In some instances, the alloy has a chemical composition within a range of 0.17-0.23 carbon, 0.30-0.60 manganese, 0.30 maximum (max) silicon, 0.10 max chromium, 0.003 max niobium, 0.003 max titanium, 0.003 vanadium, 0.10 max molybdenum, 0.10 max nickel, 0.015 max sulfur, 0.025 max phosphorus, 0.012 max nitrogen, 0.001 max boron, and 0.015-0.065 aluminum. An alloy having such a chemical composition is hot rolled to produce a hot rolled steel strip that has a thickness of less than 10 millimeters (mm). The hot rolled steel strip is then cold rolled to produce a cold rolled sheet that has a thickness that is less than 50% of the hot rolled steel strip thickness. In addition, the cold rolled sheet is recovery annealed such that the recovery annealed sheet having the above-identified chemical composition has a yield strength greater than 550 megapascals (MPa) and a percent elongation to failure greater than 3.5%.

The steel alloy is hot rolled using a roughing treatment at temperatures between 950 and 1350° C., and in some instances between 1100 and 1350° C., and a finishing treatment with an entry temperature between 950 and 1100° C. and an exit temperature between 780 and 920° C. The hot rolled strip has a thickness between 1.5 and 6.0 mm and can be coiled at temperatures between 500 and 730° C. In some instances, the hot rolled strip can be coiled at temperatures between 500 and 680° C., in the alternative between 520 and 730° C., or in another alternative between 520 and 680° C.

After the cold rolling, the cold rolled sheet has a thickness between 0.3 and 2.3 mm. In addition, the cold rolled steel sheet is recovery annealed at temperatures between 500 and 620° C. or in the alternative between 500 and 580° C. The microstructure of the recovery annealed cold rolled steel sheet has less than 10 volume percent recrystallized grains, or in the alternative less than 5 volume percent recrystallized grains. In still another alternative, the microstructure of the recovery annealed cold rolled steel sheet has less than 2 volume percent recrystallized grains.

In some instances, the steel alloy is in the form of a steel slab with a thickness between 50 and 280 mm which is hot rolled in the roughing treatment to produce a transfer bar with a thickness between 45 and 70 mm. The transfer bar is hot rolled during the finishing treatment at temperatures between 780 and 1100° C. to produce the hot rolled steel strip with the thickness between 1.5 and 6.0 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical plot of yield strength versus head, middle, and tail locations for a plurality of recovery annealed cold rolled steel sheet coils (hereafter plurality of coils) produced according to an embodiment of the present invention;

FIG. 2 is a graphical plot of tensile strength for head, middle, and tail locations for a plurality of coils produced according to an embodiment of the present invention;

FIG. 3 is a graphical plot of percent elongation to fracture for head, middle, and tail locations for a plurality of coils produced according to an embodiment of the present invention;

FIG. 4 is a graphical plot of hardness for head, middle, and tail locations for a plurality of coils produced according to an embodiment of the present invention;

FIG. 5 is a graphical plot of yield strength versus recovery annealing temperature for a plurality of coils produced according to an embodiment of the present invention;

FIG. 6 is a graphical plot of tensile strength versus recovery annealing temperature for a plurality of coils produced according to an embodiment of the present invention;

FIG. 7 is a graphical plot of elongation to fracture versus recovery annealing temperature for a plurality of coils produced according to an embodiment of the present invention;

FIG. 8 is a graphical plot of yield strength versus continuous annealing line speed for a plurality of coils produced according to an embodiment of the present invention;

FIG. 9 is a graphical plot of tensile strength versus continuous annealing line speed for a plurality of coils produced according to an embodiment of the present invention;

FIG. 10 is a graphical plot for percent elongation to fracture versus continuous annealing line speed for a plurality of coils produced according to an embodiment of the present invention; and

FIG. 11 is a series of optical micrographs illustrating a recovered but non-recrystallized grain structure for material produced according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A cold rolled mild carbon steel sheet having a 0.2% yield strength greater than 550 MPa and a percent elongation to failure greater than 3.5% is provided. A process for producing such a cold rolled mild steel sheet is also provided.

The mild carbon steel alloy can be aluminum-killed or silicon-killed and have a chemical composition in weight percent (wt %) within the range of 0.12-0.25 carbon (C), 0.30-1.70 manganese (Mn), 0.50 maximum (max) silicon (Si), 0.10 max chromium (Cr), 0.01 max niobium (Nb), 0.035 max titanium (Ti), 0.01 max vanadium (V), 0.10 max molybdenum (Mo), 0.10 max nickel (Ni), 0.015 max sulfur (S), 0.025 max phosphorus (P), 0.012 max nitrogen (N), 0.003 max boron (B), 0.015-0.065 aluminum (Al) and the balance iron (Fe). In some instances, the alloy can have a chemical composition within the range of 0.17-0.23 C, 0.30-0.60 Mn, 0.3 max Si, 0.10 max Cr, 0.003 max Nb, 0.003 max Ti, 0.003 max V, 0.10 max Mo, 0.10 max Ni, 0.015 max S, 0.025 max P, 0.012 max N, 0.001 max B, 0.15-0.065 Al and the balance Fe. In addition, incidental impurities known to those skilled in the art with respect to production of steels can be present within the alloy.

A cold rolled mild carbon steel sheet with the above-identified chemical composition and mechanical properties is manufactured/produced by casting a steel alloy slab with a desired composition and a thickness between 50 and 280 millimeters (mm). The slab is soaked at a temperature between 1100 and 1350° C. and then hot rolled with a roughing treatment at temperatures between 950 and 1350° C. In some instances, the slab is soaked at a temperature between 1100 and 1270° C., or in the alternative between 1215 and 1350° C., or in another alternative between 1215 and 1270° C., and the roughing treatment occurs at temperatures between 1100 and 1350° C.

The roughing treatment can produce a transfer bar having a thickness between 45 and 70 mm which can be subjected to a finishing treatment in which additional hot rolling is performed. The transfer bar can enter the finishing treatment at an entry temperature between 950 and 1100° C. and exit the finishing treatment at a temperature between 780 and 920° C. Upon exiting the finishing treatment, the steel is in the form of hot rolled sheet (aka hot rolled strip) which can be formed or wound into a coil, i.e. coiled, at a temperature between 500 and 730° C. In some instances, the hot rolled strip can be coiled at temperatures between 500 and 680° C., in the alternative between 520 and 730° C., or in another alternative between 520 and 680° C. In addition, the hot rolled strip can have a thickness between the range of 1.5 to 6.0 mm.

The coiled hot rolled strip is cold rolled with a reduction in sheet thickness ranging between 50 to 75%. The cold rolled sheet is then annealed in a continuous annealing line (CAL) at a temperature between 500 and 620° C. and a CAL speed between 40 and 200 meters per minute (m/min). In some instances, the cold rolled sheet is annealed in the CAL at a temperature between 500 and 580° C. and/or a CAL speed between 70 and 200 m/min. The above described process can produce a cold rolled sheet having a thickness between 0.30 and 2.3 mm that is recovery annealed with a microstructure that is either void or mostly void of recrystallization.

The recovery annealed cold rolled mild steel sheet can have a 0.2% yield strength of greater than 550 MPa, a tensile strength greater than 550 MPa, a ductility measured by percent elongation at failure of greater than 3.5%, and/or a Rockwell B hardness (HRB) greater than 85. In addition, the ratio of yield strength to tensile strength can be between 0.50 and 1.0. In addition, the recovery annealed cold rolled steel sheet can be subjected to a skin pass or temper rolling treatment It is appreciated that the temper rolling process can enhance properties of the cold rolled sheet through cold forming of the steel product in the bite of the work rolls. The properties that are typically enhanced by a temper pass due to elongation of the product include dimensional trueness and repeatability, suppression of yield point elongation, improved product surface finish, improved product shape and flatness, decreased coil memory, increased product yield strength and development of proper stiffness or temper.

In order to provide specific examples of the invention and yet not limit the scope thereof in any way, two examples of a composition and a process according to an embodiment of the present invention are provided below.

Examples

Two coils were produced from two separate slabs, each slab having thickness of approximately 250 mm and a chemical composition within the range provided above. A first slab was soaked at 1230° C. and then subjected to a hot rolling roughing treatment. The roughing treatment provided a transfer bar with a thickness of 48 mm, which was then subjected to a hot rolling finishing treatment. The temperature at the entry to the finishing treatment for the first slab was 1035° C. and the temperature at the exit of the finishing treatment was 875° C. Upon exiting the finishing treatment, hot strip with a thickness between 1.5 and 6.0 mm was coiled at 580° C. The hot strip coil was then uncoiled and cold rolled to produce a 62% reduction in thickness ({[original thickness−final thickness]/original thickness}×100) and recovery annealed at 520° C. with a CAL having a 100 m soak/annealing zone and a line speed of 70 m/min. The cold rolled and recovery annealed sheet from the first slab had a thickness of 0.9 mm, a 0.2% yield strength of 620 MPa, a tensile strength of 820 MPa, a percent elongation to failure of 5.50% and a hardness of 90 HRB.

A second slab was soaked at 1240° C. and then subjected to a roughing treatment to produce a transfer bar. The transfer bar had an entry temperature for the finishing treatment of 1044° C. while the exit temperature for the finishing treatment was 900° C. The hot rolled strip was then coiled at 630° C. and subsequently cold rolled 72% to produce a cold rolled sheet having a thickness of 1.1 mm. Thereafter, the cold rolled sheet was recovery annealed at 570° C. in a CAL having a 100 m soak/annealing zone and a line speed of 180 m/min. The cold rolled and recovery annealed mild carbon steel sheet had a 0.2% yield strength of 650 MPa, a tensile strength of 820 MPa, a ductility of 13% and a hardness of 98 HRB.

In order to test the variation of properties at different coil locations, the yield strength, tensile strength, elongation, and hardness were measured for samples taken from the head, middle, and tail locations of coils produced according to an embodiment of the present invention. The results of the testing are shown in FIGS. 1-4 with the data illustrating that material from the various locations of the coils provided properties that meet or exceed the requirements of the material discussed above, i.e. 0.2% yield strength greater than 550 MPa, tensile strength greater than 550 MPa, percent elongation greater than 3.5%, and hardness greater than 85 HRB. As shown in FIGS. 1-4, in some instances the 0.2 yield strength is greater than 600 MPa, the tensile strength is greater than 700 MPa, the percent elongation to failure is greater than 4% and the hardness is greater than 90. In addition, FIGS. 5 and 6 illustrate data for yield strength and tensile strength, respectively, of various coils as a function of annealing temperature.

Turning now to FIG. 7, percent elongation to failure as a function of annealing temperature is shown while FIGS. 8-10 provide data for 0.2% yield strength, tensile strength, and percent elongation, respectively, as a function of continuous annealing line speed. Finally, FIG. 11 provides a series of optical micrographs that show the microstructure of recovered but non-recrystallized grains for a sample taken from a coil having a chemical composition and subjected to the inventive process disclosed above. The microstructure is seen to be visually void of recrystallized grains. In some instances, the microstructure is mostly void of recrystallized grains, e.g. less than 10 volume percent (vol %) of recrystallized grains, preferably less than 5 vol % recrystallized grains, and more preferably less than 2 vol % recrystallized grains. In addition, such properties as disclosed herein are appreciated to be unknown in the prior art for such a mild steel and thus unexpected results have been demonstrated.

In view of the teaching presented herein, it is to be understood that numerous modifications and variations of the present invention will be readily apparent to those of skill in the art. The foregoing is illustrative of specific embodiments of the invention, but is not meant to be a limitation upon the practice thereof. It is the claims, and all equivalents thereof, that define the scope of the invention. 

We claim:
 1. A process for making a high strength steel comprising: providing a steel alloy having a chemical composition in weight percent within a range of 0.12-0.25 carbon, 0.30-1.70 manganese, 0.50 maximum (max) silicon, 0.10 max chromium, 0.01 max niobium, 0.035 max titanium, 0.01 vanadium, 0.10 max molybdenum, 0.10 max nickel, 0.015 max sulfur, 0.025 max phosphorus, 0.012 max nitrogen, 0.003 max boron, 0.015-0.065 aluminum, balance iron and incidental impurities; hot rolling the steel alloy to produce a hot rolled strip having a thickness of less than 10 mm; cold rolling the hot rolled strip to produce a cold rolled sheet having a thickness less than 50% of the hot rolled strip thickness; and recovery annealing the cold rolled sheet, the recovery annealed cold rolled sheet having a yield strength greater than 550 MPa and a percent elongation to failure greater than 3.5%.
 2. The process of claim 1, wherein the steel alloy is hot rolled using a roughing treatment at temperatures between 950 and 1350° C. and a finishing treatment having an entry temperature between 950 and 1100° C. and an exit temperature between 780 and 920° C.
 3. The process of claim 2, wherein the hot rolled strip has a thickness between 1.5 and 6.0 mm.
 4. The process of claim 3, further including coiling of the hot rolled strip at temperatures between 500 and 730° C.
 5. The process of claim 4, wherein the cold rolled sheet has a thickness between 0.3 and 2.3 mm.
 6. The process of claim 5, wherein the cold rolled sheet is recovery annealed at temperatures between 500 and 620° C.
 7. The process of claim 6, wherein a microstructure of the recovery annealed cold rolled sheet has less than 10 volume percent recrystallized grains.
 8. The process of claim 7, wherein the microstructure of the recovery annealed cold rolled sheet has less than 5 volume percent recrystallized grains.
 9. The process of claim 8, wherein the microstructure of the recovery annealed cold rolled sheet has less than 2 volume percent recrystallized grains.
 10. The process of claim 1, further including temper rolling the recovery annealed cold rolled sheet between 0.0 and 5.0%.
 11. A process for making a high strength steel sheet comprising: providing a steel slab with a thickness between 50 and 280 mm, the steel slab having a chemical composition in weight percent within a range of 0.12-0.25 carbon, 0.30-1.70 manganese, 0.50 maximum (max) silicon, 0.10 max chromium, 0.01 max niobium, 0.035 max titanium, 0.01 vanadium, 0.10 max molybdenum, 0.10 max nickel, 0.015 max sulfur, 0.025 max phosphorus, 0.012 max nitrogen, 0.003 max boron, 0.015-0.065 aluminum, balance iron and incidental impurities; soaking the steel slab at a temperatures between 1100 and 1350° C.; hot rolling the steel at temperatures between 780 and 1100° C. and producing a hot rolled strip with a thickness between 1.5 and 6.0 mm; coiling the hot rolled strip at temperatures between 500 and 730° C.; cold rolling the hot rolled strip to produce a cold rolled sheet having a thickness between 0.30 and 2.3 mm; and recovery annealing the cold rolled sheet at temperatures between 500 and 620° C., the recovery annealed cold rolled sheet having a yield strength greater than 550 MPa and a percent elongation to failure greater than 3.5%.
 12. The process of claim 11, wherein the hot rolling includes a roughing treatment that produces a transfer bar with a thickness between 45 and 70 mm.
 13. The process of claim 12, wherein the hot rolling includes a finishing treatment at temperatures between 780 and 920° C. and produces the hot rolled strip with a thickness between 1.5 and 6.0 mm.
 14. The process of claim 11, further including temper rolling the recovery annealed cold rolled sheet between 0.0 and 5.0%.
 15. A high strength-high ductility cold rolled steel comprising: a steel alloy having a chemical composition in weight percent within a range of 00.12-0.25 carbon, 0.30-1.70 manganese, 0.50 maximum (max) silicon, 0.10 max chromium, 0.01 max niobium, 0.035 max titanium, 0.01 vanadium, 0.10 max molybdenum, 0.10 max nickel, 0.015 max sulfur, 0.025 max phosphorus, 0.012 max nitrogen, 0.003 max boron, 0.015-0.065 aluminum, balance iron and incidental impurities; said steel alloy having a recovery annealed microstructure, a yield strength greater than 550 MPa and a percent elongation to failure greater than 3.5%.
 16. The high strength-high ductility cold rolled steel of claim 15, wherein said steel alloy has Rockwell B hardness greater than
 100. 17. The high strength-high ductility cold rolled steel of claim 16, wherein said steel alloy exhibits a yield strength-to-tensile strength ratio between 0.25 and 1.00.
 18. The high strength-high ductility cold rolled steel of claim 15, wherein said recovery annealed microstructure has less than 5 volume percent of recrystallized grains.
 19. The high strength-high ductility cold rolled steel of claim 18, wherein said recovery annealed microstructure has less than 2 volume percent of recrystallized grains. 